U.S. patent application number 10/552920 was filed with the patent office on 2006-08-31 for nonaqueous electrolyte secondary battery and charge/discharge system thereof.
Invention is credited to So Kuranaka, Masatoshi Nagayama, Shoichiro Watanabe.
Application Number | 20060194109 10/552920 |
Document ID | / |
Family ID | 33447311 |
Filed Date | 2006-08-31 |
United States Patent
Application |
20060194109 |
Kind Code |
A1 |
Watanabe; Shoichiro ; et
al. |
August 31, 2006 |
Nonaqueous electrolyte secondary battery and charge/discharge
system thereof
Abstract
A non-aqueous electrolyte secondary battery including: a
positive electrode including a positive electrode material mixture
layer; a negative electrode including a negative electrode material
mixture layer; a separator or lithium-ion conductive porous film
interposed between the positive electrode and the negative
electrode; and a lithium-ion conductive non-aqueous electrolyte.
The positive electrode material mixture layer contains a positive
electrode active material comprising a lithium transition metal
composite oxide, and the lithium transition metal composite oxide
contains lithium, a transition metal, and a metal different from
the transition metal. The negative electrode material mixture layer
contains a negative electrode active material comprising a carbon
material. In the area where the positive electrode material mixture
layer and the negative electrode material mixture layer are opposed
to each other, the ratio R:Wp/Wn is 1.3 to 2.2 where Wp is the
weight of the positive electrode active material contained in the
positive electrode material mixture layer per unit opposed area and
Wn is the weight of the negative electrode active material
contained in the negative electrode material mixture layer per unit
opposed area. The end of charge voltage is set to 4.25 to 4.5 V in
normal operation.
Inventors: |
Watanabe; Shoichiro;
(Ikoma-gun, JP) ; Nagayama; Masatoshi;
(Hirakata-shi, JP) ; Kuranaka; So; (Osaka-shi,
JP) |
Correspondence
Address: |
PANASONIC PATENT CENTER;c/o MCDERMOTT WILL & EMERY LLP
600 13TH STREET, NW
WASHINGTON
DC
20005-3096
US
|
Family ID: |
33447311 |
Appl. No.: |
10/552920 |
Filed: |
May 11, 2004 |
PCT Filed: |
May 11, 2004 |
PCT NO: |
PCT/JP04/06620 |
371 Date: |
October 11, 2005 |
Current U.S.
Class: |
429/231.1 ;
429/223; 429/224; 429/231.3; 429/231.4; 429/231.5; 429/231.6 |
Current CPC
Class: |
H01M 4/505 20130101;
H01M 4/525 20130101; H01M 4/131 20130101; H01M 4/133 20130101; H01M
4/485 20130101; H01M 10/448 20130101; H01M 2010/4292 20130101; H01M
10/0587 20130101; H01M 10/0525 20130101; Y02E 60/10 20130101 |
Class at
Publication: |
429/231.1 ;
429/231.4; 429/231.3; 429/231.6; 429/223; 429/224; 429/231.5 |
International
Class: |
H01M 4/48 20060101
H01M004/48; H01M 4/58 20060101 H01M004/58; H01M 4/50 20060101
H01M004/50; H01M 4/52 20060101 H01M004/52 |
Foreign Application Data
Date |
Code |
Application Number |
May 16, 2003 |
JP |
2003-138849 |
Claims
1. A non-aqueous electrolyte secondary battery comprising: a
positive electrode comprising a positive electrode substrate and a
positive electrode material mixture layer carried on said positive
electrode substrate; a negative electrode comprising a negative
electrode substrate and a negative electrode material mixture layer
carried on said negative electrode substrate; a separator or
lithium-ion conductive porous film interposed between said positive
electrode and said negative electrode; and a lithium-ion conductive
non-aqueous electrolyte, wherein said positive electrode material
mixture layer comprises a positive electrode active material
comprising a lithium transition metal composite oxide, said lithium
transition metal composite oxide comprising lithium, a transition
metal, and a metal different from said transition metal, said
negative electrode material mixture layer comprises a negative
electrode active material comprising a carbon material that is
capable of absorbing and desorbing lithium, the end of charge
voltage of said non-aqueous electrolyte secondary battery is set to
4.25 to 4.5 V in normal operation, and the ratio R:Wp/Wn is 1.3 to
2.2 in the area where said positive electrode material mixture
layer and said negative electrode material mixture layer are
opposed to each other, said Wp being the weight of the positive
electrode active material contained in said positive electrode
material mixture layer per unit opposed area, said Wn being the
weight of the negative electrode active material contained in said
negative electrode material mixture layer per unit opposed
area.
2. The non-aqueous electrolyte secondary battery in accordance with
claim 1, wherein said lithium transition metal composite oxide is
represented by the general formula (1):
Li.sub.xCo.sub.1-yM.sub.yO.sub.2, said general formula (1)
satisfies 1.0.ltoreq.x.ltoreq.1.03 and 0.005.ltoreq.y.ltoreq.0.15,
the element M in said general formula (1) is at least one selected
from the group consisting of Mg, Al, Ti, Sr, Mn, Ni and Ca, and
said ratio R is 1.5 to 2.2.
3. The non-aqueous electrolyte secondary battery in accordance with
claim 1, wherein said lithium transition metal composite oxide is
represented by the general formula (2):
Li.sub.xNi.sub.yMn.sub.zM.sub.1-y-zO.sub.2, said general formula
(2) satisfies 1.0.ltoreq.x.ltoreq.1.03, 0.3.ltoreq.y.ltoreq.0.5,
0.3.ltoreq.z.ltoreq.0.5, and 0.9.ltoreq.y/z.ltoreq.1.1, the element
M in said general formula (2) is at least one selected from the
group consisting of Co, Mg, Al, Ti, Sr and Ca, and said ratio R is
1.3 to 2.0.
4. The non-aqueous electrolyte secondary battery in accordance with
claim 1, wherein said lithium transition metal composite oxide
comprises a composite oxide A and a composite oxide B, said
composite oxide A is represented by the general formula (1):
Li.sub.xCo.sub.1-yM.sub.yO.sub.2, said general formula (1)
satisfies 1.0.ltoreq.x.ltoreq.1.03, and 0.005.ltoreq.y.ltoreq.0.15,
the element M in said general formula (1) is at least one selected
from the group consisting of Mg, Al, Ti, Sr, Mn, Ni and Ca, said
composite oxide B is represented by the general formula (2):
Li.sub.xNi.sub.yMn.sub.zM.sub.1-y-zO.sub.2, said general formula
(2) satisfies 1.0.ltoreq.x.ltoreq.1.03, 0.3.ltoreq.y.ltoreq.0.5,
0.3.ltoreq.z.ltoreq.0.5, and 0.9.ltoreq.y/z.ltoreq.1.1, the element
M in said general formula (2) is at least one selected from the
group consisting of Co, Mg, Al, Ti, Sr and Ca, and said ratio R is
1.3 to 2.2.
5. The non-aqueous electrolyte secondary battery in accordance with
claim 4, wherein the weight ratio between said composite oxide A
and said composite oxide B is 9:1 to 1:9.
6. The non-aqueous electrolyte secondary battery in accordance with
claim 1, wherein said positive electrode material mixture layer
contains a metal oxide represented by the general formula (3):
MO.sub.x, said general formula (3) satisfies
0.4.ltoreq.x.ltoreq.2.0, and the element M in said general formula
(3) is at least one selected from the group consisting of Li, Co,
Mg, Al, Ti, Sr, Mn, Ni and Ca.
7. A charge and discharge system for a non-aqueous electrolyte
secondary battery, comprising the non-aqueous electrolyte secondary
battery as recited in claim 1 and a charger for said non-aqueous
electrolyte secondary battery, wherein said charger is set such
that it stops charging when the voltage of said secondary battery
reaches 4.25 to 4.5 V.
Description
TECHNICAL FIELD
[0001] The present invention relates to a non-aqueous electrolyte
secondary battery utilizing lithium ions, and particularly, to a
non-aqueous electrolyte secondary battery that operates at high
voltage.
BACKGROUND ART
[0002] Recently, non-aqueous electrolyte secondary batteries used
as the main power source for mobile communications appliances and
portable electronic appliances have high electromotive force and
high energy density. The positive electrode of non-aqueous
electrolyte secondary batteries usually comprises a lithium
transition metal composite oxide as a positive electrode active
material. Among lithium transition metal composite oxides, lithium
cobalt oxide (LiCoO.sub.2), lithium nickel oxide (LiNiO.sub.2), and
the like are preferable. These lithium transition metal composite
oxides have a potential of 4 V or more relative to lithium.
[0003] In the case of non-aqueous electrolyte secondary batteries
utilizing lithium ions (lithium ion secondary batteries), if the
end of charge voltage of the battery is heightened, the capacity is
increased commensurately. Hence, heightening the operating voltage
of non-aqueous electrolyte secondary batteries is examined.
[0004] For example, in non-aqueous electrolyte secondary batteries
including a manganese-containing spinel lithium oxide as a positive
electrode active material, there has been a proposal to set the
upper limit charge voltage in the range of 4.0 V to 4.5 V. Spinel
lithium oxides are stable even at high potential (see Japanese
Laid-Open Patent Publication No. 2001-307781).
[0005] Predominant non-aqueous electrolyte secondary batteries
including a lithium cobalt oxide as a positive electrode active
material have a high capacity and excellent characteristics such as
cycle characteristics and storage characteristics. However, if such
non-aqueous electrolyte secondary batteries are repeatedly charged
up to high voltage and discharged, their capacity and the thermal
stability of the active material degrade. Thus, the conventional
end of charge voltage in normal operation is 4.2 V at most, and
even if control circuit errors are allowed for, it is less than
4.25 V at most. If non-aqueous electrolyte secondary batteries are
operated at a voltage of 4.25 V or higher, their safety may be
particularly impaired.
[0006] Even in the case of the end of charge voltage being set to
4.2 V, if the battery is overcharged, for example, accidentally,
the battery voltage increases to more than that. In such cases, it
is also desired that the positive electrode active material
maintain its stable crystal structure. Thus, there has been
proposed a technique by which a specific element is incorporated in
the form of solid solution in a composite oxide constituting the
positive electrode active material (see Japanese Laid-Open Patent
Publication No. 2002-203553).
[0007] Further, there has also been a proposal to use a mixture of
specific two kinds of composite oxides as a positive electrode
active material, in order to improve the thermal stability of the
battery upon overcharge (see Japanese Laid-Open Patent Publication
No. 2002-319398).
DISCLOSURE OF INVENTION
[0008] In the case of using a positive electrode active material
that is stable at high voltage and setting the end of charge
voltage of a non-aqueous electrolyte secondary battery to 4.25 V or
higher in normal operation, the utilization rate of the positive
electrode improves and the battery capacity increases. However,
this causes a change in the relation between the utilization rate
of the positive electrode and the load on the negative electrode.
Therefore, if the conventional battery design in which the end of
charge voltage is set to 4.2 V is employed as it is, the capacity
balance between the positive electrode and the negative electrode
is destroyed, thereby causing a problem.
[0009] It is therefore an object of the present invention to
provide a high-capacity non-aqueous electrolyte secondary battery
that operates normally even if the end of charge voltage is set to
4.25 V or higher in normal operation. That is, the present
invention intends to provide a non-aqueous electrolyte secondary
battery capable of securing safety, charge/discharge cycle
characteristics, heat resistance, storage characteristics, etc.,
even if the end of charge voltage is set to 4.25 V or higher in
normal operation.
[0010] In order to maintain the capacity balance between the
positive electrode and the negative electrode while increasing the
battery capacity, it is effective to reduce the weight of the
positive electrode active material and increase the weight of the
negative electrode active material while setting the end of charge
voltage to 4.25 V or higher in normal operation. It should be
noted, however, that the degree of contribution of the active
material to charge and discharge locally varies depending on the
electrode position. Therefore, the positional relation between the
positive electrode active material and the negative electrode
active material also needs to be taken into consideration.
[0011] The present invention is achieved in view the above
circumstances and relates to a non-aqueous electrolyte secondary
battery including: a positive electrode comprising a positive
electrode substrate and a positive electrode material mixture layer
carried on the positive electrode substrate; a negative electrode
comprising a negative electrode substrate and a negative electrode
material mixture layer carried on the negative electrode substrate;
a separator or lithium-ion conductive porous film interposed
between the positive electrode and the negative electrode; and a
lithium-ion conductive non-aqueous electrolyte. The positive
electrode material mixture layer comprises a positive electrode
active material comprising a lithium transition metal composite
oxide, the lithium transition metal composite oxide containing
lithium, a transition metal, and a metal different from the
transition metal. The negative electrode material mixture layer
comprises a negative electrode active material comprising a carbon
material that is capable of absorbing and desorbing lithium. The
end of charge voltage of the non-aqueous electrolyte secondary
battery is set to 4.25 to 4.5 V in normal operation. In the area
where the positive electrode material mixture layer and the
negative electrode material mixture layer are opposed to each
other, the ratio R:Wp/Wn is 1.3 to 2.2 where Wp is the weight of
the positive electrode active material contained in the positive
electrode material mixture layer in unit opposed area and Wn is the
weight of the negative electrode active material contained in the
negative electrode material mixture layer in unit opposed area.
[0012] When the lithium transition metal composite oxide is
represented by the general formula (1):
Li.sub.xCo.sub.1-yM.sub.yO.sub.2, where the general formula (1)
satisfies 1.0.ltoreq.x.ltoreq.1.03 and 0.005.ltoreq.y.ltoreq.0.15
and the element M in the general formula (1) is at least one
selected from the group consisting of Mg, Al, Ti, Sr, Mn, Ni and
Ca, it is preferred that the ratio R be 1.5 to 2.2.
[0013] When the lithium transition metal composite oxide is
represented by the general formula (2):
Li.sub.xNi.sub.yMn.sub.zM.sub.1-y-zO.sub.2, where the general
formula (2) satisfies 1.0.ltoreq.x.ltoreq.1.03,
0.3.ltoreq.y.ltoreq.0.5, 0.3.ltoreq.z.ltoreq.0.5, and
0.9.ltoreq.y/z.ltoreq.1.1 and the element M in the general formula
(2) is at least one selected from the group consisting of Co, Mg,
Al, Ti, Sr and Ca, it is preferred that the ratio R be 1.3 to
2.0.
[0014] When the lithium transition metal composite oxide comprises
a composite oxide A and a composite oxide B, the composite oxide A
is represented by the general formula (1):
Li.sub.xCo.sub.1-yM.sub.yO.sub.2, where the general formula (1)
satisfies 1.0.ltoreq.x.ltoreq.1.03 and 0.005.ltoreq.y.ltoreq.0.15
and the element M in the general formula (1) is at least one
selected from the group consisting of Mg, Al, Ti, Sr, Mn, Ni and
Ca, and the composite oxide B is represented by the general formula
(2): Li.sub.xNi.sub.yMn.sub.zM.sub.1-y-zO.sub.2, where the general
formula (2) satisfies 1.0.ltoreq.x.ltoreq.1.03,
0.3.ltoreq.y.ltoreq.0.5, 0.3.ltoreq.z.ltoreq.0.5, and
0.9.ltoreq.y/z.ltoreq.1.1 and the element M in the general formula
(2) is at least one selected from the group consisting of Co, Mg,
Al, Ti, Sr and Ca, it is preferred that the ratio R be 1.3 to
2.2.
[0015] When the lithium transition metal composite oxide comprises
the composite oxide A and the composite oxide B, it is preferred
that the weight ratio between the composite oxide A and the
composite oxide B be 9:1 to 1:9.
[0016] In the non-aqueous electrolyte secondary battery in
accordance with the present invention, the positive electrode
material mixture layer can contain a metal oxide represented by the
general formula (3): MO.sub.x, in addition to the positive
electrode active material. It is preferred that the general formula
(3) satisfy 0.4.ltoreq.x.ltoreq.2.0 and the element M in the
general formula (3) be at least one selected from the group
consisting of Li, Co, Mg, Al, Ti, Sr, Mn, Ni and Ca.
[0017] The present invention also relates to a charge and discharge
system for a non-aqueous electrolyte secondary battery, including
the above-mentioned non-aqueous electrolyte secondary battery and a
charger therefor. The charger is set such that it stops charging
when the voltage of the non-aqueous electrolyte secondary battery
reaches 4.25 to 4.5 V.
BRIEF DESCRIPTION OF DRAWINGS
[0018] FIG. 1 is a partially cut-away perspective view of an
exemplary non-aqueous electrolyte secondary battery of the present
invention.
BEST MODE FOR CARRYING OUT THE INVENTION
[0019] The present invention relates to a non-aqueous electrolyte
secondary battery whose end of charge voltage is set to 4.25 to 4.5
V in normal operation. The non-aqueous electrolyte secondary
battery of the present invention maintains sufficient safety and
operates normally even if it is used with the end of charge voltage
set to, for example, 4.30 V or higher, 4.35 V or higher, 4.40 V or
higher, or 4.45 V or higher in normal operation.
[0020] Thus, in a charge and discharge system including the
non-aqueous electrolyte secondary battery of the present invention
and a charger therefor, when the voltage of the non-aqueous
electrolyte secondary battery reaches 4.25 to 4.5 V, the charging
is stopped. Such a system is preferable as the power supply system
for devices such as cellular phones and personal computers.
[0021] As used herein, normal operation refers to a state of normal
operation of a non-aqueous electrolyte secondary battery, or a
state of operation recommended by the manufacturer of the
battery.
[0022] Also, the end of charge voltage refers to a reference
voltage at which a constant current charge of a battery is stopped,
and when the voltage of a battery that is being charged reaches the
reference voltage, the constant current charge of the battery is
stopped. Thereafter, usually, at this reference voltage a constant
voltage charge is performed. The end of charge voltage is
determined in advance depending on the design of the non-aqueous
electrolyte secondary battery.
[0023] The end of charge voltage in normal operation is usually a
preferable voltage for a non-aqueous electrolyte secondary battery
to operate normally, or the upper limit voltage in the recommended
battery voltage range.
[0024] The non-aqueous electrolyte secondary battery according to
the present invention includes: a positive electrode including a
positive electrode substrate and a positive electrode material
mixture layer carried on the positive electrode substrate; a
negative electrode including a negative electrode substrate and a
negative electrode material mixture layer carried on the negative
electrode substrate; a separator or lithium-ion conductive porous
film interposed between the positive electrode and the negative
electrode; and a lithium-ion conductive non-aqueous
electrolyte.
[0025] The positive electrode substrate and the negative electrode
substrate may be made of any conventionally known material without
any particular limitation.
[0026] The positive electrode material mixture layer and the
negative electrode material mixture layer can contain, without any
particular limitation, conventionally known electrode materials, in
addition to a positive electrode active material and a negative
electrode active material. Exemplary electrode materials include,
for example, a conductive agent and a binder.
[0027] The positive electrode material mixture layer according to
the present invention includes a positive electrode active material
comprising a lithium transition metal composite oxide. Also, the
negative electrode material mixture layer according to the present
invention includes a negative electrode active material comprising
a carbon material capable of absorbing and desorbing lithium.
[0028] In the area where the positive electrode material mixture
layer and the negative electrode material mixture layer are opposed
to each other, the ratio R:Wp/Wn is 1.3 to 2.2 where Wp is the
weight of the positive electrode active material contained in the
positive electrode material mixture layer per unit opposed area and
Wn is the weight of the negative electrode active material
contained in the negative electrode material mixture layer per unit
opposed area.
[0029] The lithium transition metal composite oxide forming the
positive electrode active material contains lithium, a transition
metal as a main component, and a small amount of a metal different
from the above-mentioned transition metal. The addition of the
different metal enhances the stability of the crystal structure of
the lithium transition metal composite oxide. As a result, it is
possible to obtain a non-aqueous electrolyte secondary battery that
operates normally even if the end of charge voltage is set to a
high voltage in normal operation.
[0030] The transition metal as the main component is preferably at
least one selected from the group consisting of Co, Mn and Ni. When
the transition metal as the main component is represented by
M.sup.1 and the small amount of the different metal is represented
by M.sup.2, the lithium transition metal composite oxide forming
the positive electrode active material can be represented, for
example, by the formula:
Li.sub.xM.sup.1.sub.1-yM.sup.2.sub.yO.sub.2
(1.0.ltoreq.x.ltoreq.1.03, 0.005.ltoreq.y.ltoreq.0.15). In
non-aqueous electrolyte secondary batteries including as a positive
electrode active material a lithium transition metal composite
oxide that does not contain a different metal (e.g., LiCoO.sub.2),
the phase of the composite oxide changes from the hexagonal system
to the monoclinic system when the battery voltage is around 4.2 V
to 4.45 V. When the battery voltage is around 4.2 V, the potential
of the positive electrode is around 4.25 V relative to metal Li. If
the battery is further charged, the phase of the composite oxide
changes into the hexagonal system, and if the battery voltage
reaches around 4.6 V, the monoclinic system appears again. The
monoclinic crystal structure appears when the whole crystal is
distorted. In such a distorted monoclinic composite oxide, the
bonding between the oxygen ion which plays the central role in
maintaining the crystal structure and the surrounding metal ions is
weak. Thus, the heat resistance of the composite oxide decreases
significantly.
[0031] Specific examples of the lithium transition metal composite
oxide forming the positive electrode active material is hereinafter
described.
[0032] In the present invention, lithium transition metal composite
oxides represented by the general formula (1):
Li.sub.xCo.sub.1-yM.sub.yO.sub.2 (hereinafter referred to as
composite oxides A) can be preferably used as positive electrode
active materials. The general formula (1) satisfies
1.0.ltoreq.x.ltoreq.1.03 and 0.005.ltoreq.y.ltoreq.0.15.
[0033] Although the value x changes with charging and discharging
of the battery, the composite oxides A as the positive electrode
raw materials (i.e., lithium transition metal composite oxides
represented by the general formula (1) immediately after the
synthesis thereof) satisfy 1.0.ltoreq.x.ltoreq.1.03.
[0034] When the general formula (1) satisfies
1.0.ltoreq.x.ltoreq.1.03, the baking of raw materials of such a
composite oxide A at high temperatures can be performed
advantageously upon synthesis of the composite oxide A.
Specifically, since the occurrence of shortage of lithium in the
composite oxide A is suppressed, the structural stability of the
composite oxide A is enhanced.
[0035] If x exceeds 1.03, lithium becomes excessive, and the
composite oxide A becomes relatively strongly alkaline. As a
result, the stability of the composite oxide A in positive
electrode preparation may be impaired, or the positive electrode
substrate made of, for example, Al may be corroded. If x is 1.0 or
more, the effect of suppressing the occurrence of lithium shortage
can be obtained, but it is particularly preferred that x be 1.01 or
more, in order to further enhance the structural stability of the
composite oxide A. On the other hand, if x is less than 1.0,
lithium becomes insufficient, so that the composite oxide A does
not provide high performance as a positive electrode active
material. That is, the content of a by-product, such as
Co.sub.3O.sub.4, contained in the composite oxide A becomes high,
thereby resulting in gas evolution due to the by-product inside the
battery, capacity degradation, etc.
[0036] In the general formula (1), the element M is at least one
selected from the group consisting of Mg, Al, Ti, Sr, Mn, Ni and
Ca. The element M contributes to stabilization of the crystal
structure of the composite oxide A. Among Mg, Al, Ti, Sr, Mn, Ni
and Ca, it is particularly preferred to use at least one selected
from the group consisting of Mg, Al and Mn. In order for the
element M to produce the effect of stabilizing the crystal
structure, at least 0.005.ltoreq.y is required to be satisfied.
However, when 0.15<y, the problem of capacity degradation of the
positive electrode active material arises.
[0037] Among the composite oxides A, particularly, lithium
transition metal composite oxides represented by the general
formula (1'): Li.sub.xCo.sub.1-y-zMg.sub.yAl.sub.zO.sub.2
(hereinafter referred to as composite oxides A') can be preferably
used. The general formula (1') satisfies 1.0.ltoreq.x.ltoreq.1.03,
0.005.ltoreq.y+z.ltoreq.0.15, and 0.001.ltoreq.z.ltoreq.0.05.
[0038] The thermal stability of the composite oxides A' is almost
the same, for example, when the positive electrode potential is 4.8
V relative to lithium metal, as when the positive electrode
potential is 4.2 V relative to lithium metal.
[0039] Although the reason why such thermal stability can be
obtained is not clear at the moment, the following is thought to be
relevant.
[0040] First, the replacement of Co with a preferable amount of Mg
increases the stability of crystal structure of a composite oxide
A' even in a state of a high degree of Li elimination due to
charging. As a result, even at high temperatures, the elimination
of oxygen from the composite oxide A' and the like are
suppressed.
[0041] Also, since the composite oxide A' has high electronic
conductivity, it is thought to function also as a conductive agent
in the positive electrode. The conductive agent contributes to
formation of a uniform potential distribution in the positive
electrode. Upon formation of a uniform potential distribution in
the positive electrode, it is thought that the amount of Co that
locally has a higher potential than the surrounding is relatively
decreased, so that the degradation in thermal stability is
suppressed.
[0042] In the general formula (1'), if x exceeds 1.03, lithium
becomes excessive, and such a composite oxide A' becomes relatively
strongly alkaline. As a result, the stability of the composite
oxide A' in positive electrode preparation may be impaired, or the
positive electrode substrate made of, for example, Al may be
corroded. Also, if x is less than 1.0, lithium becomes
insufficient, so that the composite oxide A' does not provide high
performance as a positive electrode active material. That is, the
content of a by-product, such as Co.sub.3O.sub.4, contained in the
composite oxide A' becomes high, thereby resulting in gas evolution
due to the by-product inside the battery, capacity degradation,
etc. In the general formula (1'), if y+z becomes less than 0.005,
the element M does not produce the effect of stabilizing the
crystal structure. If y+z exceeds 0.15, the capacity degradation of
the positive electrode active material becomes a problem.
[0043] On the other hand, although the reason is not clear at the
moment, Al has the effect of further strengthening the Mg's
function of improving the structural stability and heat resistance
of the composite oxides A'. This effect can be obtained if z is
0.001 or more. However, the amount of Co replaced by Al needs to be
small, and if z exceeds 0.05, the capacity of the positive
electrode may degrade significantly.
[0044] Next, in the present invention, lithium transition metal
composite oxides represented by the general formula (2):
Li.sub.xNi.sub.yMn.sub.zM.sub.1-y-zO.sub.2 (hereinafter referred to
as composite oxides B) can be preferably used as positive electrode
active materials. The general formula (2) satisfies
1.0.ltoreq.x.ltoreq.1.03, 0.3.ltoreq.y.ltoreq.0.5,
0.3.ltoreq.z.ltoreq.0.5, and 0.9.ltoreq.y/z.ltoreq.1.1.
[0045] Although the value x changes with charging and discharging
of the battery, the composite oxides B as the positive electrode
raw materials (i.e., lithium transition metal composite oxides
represented by the general formula (2) immediately after the
synthesis thereof) satisfy 1.0.ltoreq.x.ltoreq.1.03.
[0046] When the general formula (2) satisfies
1.0.ltoreq.x.ltoreq.1.03, the baking of raw materials of such a
composite oxide B at high temperatures can be performed
advantageously upon synthesis of the composite oxide B.
Specifically, since the occurrence of shortage of lithium in the
composite oxide B is suppressed, the structural stability of the
composite oxide B is enhanced.
[0047] In the general formula (2), if x exceeds 1.03, lithium
becomes excessive, and such a composite oxide B becomes relatively
strongly alkaline. As a result, the stability of the composite
oxide B in positive electrode preparation may be impaired, or the
positive electrode substrate made of, for example, Al may be
corroded. If x is 1.0 or more, the effect of suppressing the
occurrence of lithium shortage can be obtained, but it is
particularly preferred that x be 1.01 or more, in order to further
enhance the structural stability of the composite oxide B. On the
other hand, if x is less than 1.0, lithium becomes insufficient, so
that the composite oxide B does not provide high performance as a
positive electrode active material. That is, the content of a
by-product, such as Co.sub.3O.sub.4, contained in the composite
oxide B becomes high, thereby resulting in gas evolution due to the
by-product inside the battery, capacity degradation, etc.
[0048] The crystal structure of the composite oxides B belongs to
the hexagonal system only when y representing the Ni content and z
representing the Mn content in the general formula (2) satisfy
0.3.ltoreq.y.ltoreq.0.5, 0.3.ltoreq.z.ltoreq.0.5, and
0.9.ltoreq.y/z.ltoreq.1.1. This range is a singular range
exhibiting singular behavior in X-ray analysis and the like.
[0049] In the general formula (2), the element M is at least one
selected from the group consisting of Co, Mg, Al, Ti, Sr and Ca.
The element M contributes to stabilization of the crystal structure
of the composite oxides B. The addition of the element M increases
the stability of the composite oxides B having a high potential,
but x, y and z need to satisfy the above ranges.
[0050] A mixture of a composite oxide A and a composite oxide B can
also be used preferably as a positive electrode active material.
This is because the composite oxide A and the composite oxide B do
not interfere with each other.
[0051] In the mixture, the weight ratio between the composite oxide
A and the composite oxide B is preferably 9:1 to 1:9. If the weight
ratio is in such a range, the electronic conductivity of the
composite oxide A and the high capacity of the composite oxide B
produce complementary effects.
[0052] Next, in the non-aqueous electrolyte secondary battery of
the present invention, in the area where the positive electrode
material mixture layer and the negative electrode material mixture
layer are opposed to each other, the ratio R:Wp/Wn is set to 1.3 to
2.2 where Wp is the weight of the positive electrode active
material contained in the positive electrode material mixture layer
per unit opposed area and Wn is the weight of the negative
electrode active material contained in the negative electrode
material mixture layer per unit opposed area.
[0053] The reason why the ratio R is set to the above range is as
follows. In the non-aqueous electrolyte secondary battery of the
present invention, the positive electrode has a large load. Thus,
in the area where the positive electrode material mixture layer and
the negative electrode material mixture layer are opposed to each
other, there is a need to reduce the weight of the positive
electrode active material relative to the conventional weight, in
order to control the capacity balance between the positive
electrode and the negative electrode.
[0054] The ratio R can also be defined as the capacity ratio.
However, in actual production of batteries, a predetermined weight
of a positive electrode active material and a predetermined weight
of a negative electrode active material are measured. Thus,
employing the weight ratio is more clear.
[0055] If the ratio R becomes less than 1.3, the substantial amount
of the negative electrode active material becomes extremely
excessive relative to the amount of the positive electrode active
material in the area where the positive electrode material mixture
layer and the negative electrode material mixture layer are opposed
to each other. As a result, the thermal stability of the battery
degrades, and the safety of the battery upon exposure to high
temperatures deteriorates. Also, if the ratio R exceeds 2.2, the
negative electrode load becomes too heavy relative to the positive
electrode load. Consequently, upon repetition of charge/discharge
cycles, lithium metal may be deposited on the negative electrode,
thereby resulting in deterioration in battery reliability.
[0056] In the case of using either a composite oxide A or a
composite oxide B as a positive electrode active material, and when
using the composite oxide A, the ratio R is preferably in the range
of 1.5 to 2.2, more preferably in the range of 1.5 to 2.0, and
particularly preferably in the range of 1.5 to 1.8.
[0057] Also, in the case of using either a composite oxide A or a
composite oxide B as a positive electrode active material, and when
using the composite oxide B, the ratio R is preferably in the range
of 1.3 to 2.0, and more preferably in the range of 1.3 to 1.8.
[0058] Also, in the case of using a mixture of a composite oxide A
and a composite oxide B as a positive electrode active material,
the ratio R is preferably in the range of 1.3 to 2.2.
[0059] In the non-aqueous electrolyte secondary battery of the
present invention, the positive electrode material mixture layer
can contain a metal oxide represented by the general formula (3):
MO.sub.x, in addition to the positive electrode active material.
The general formula (3) satisfies 0.4.ltoreq.x.ltoreq.2.0, and the
element M in the general formula (3) is preferably at least one
selected from the group consisting of Li, Co, Mg, Al, Ti, Sr, Mn,
Ni and Ca.
[0060] The carbon material capable of absorbing and desorbing
lithium that forms the negative electrode active material may be
any conventionally known material without any particular
limitation. Examples include thermally decomposed carbons, cokes
such as pitch coke, needle coke, and petroleum coke, graphites,
glass carbons, materials obtained by baking phenolic resin, furan
resin, or the like at an appropriate temperature and carbonizing it
(baked organic polymer compound), carbon fibers, and active carbon.
Among them, graphites are particularly preferable.
[0061] The lithium-ion conductive non-aqueous electrolyte is
preferably a non-aqueous electrolyte comprising a non-aqueous
solvent and a lithium salt dissolved therein.
[0062] The non-aqueous solvent may be any conventionally known one
without any particular limitation. Examples include cyclic carbonic
acid esters such as ethylene carbonate (EC) and propylene carbonate
(PC), non-cyclic carbonic acid esters such as dimethyl carbonate
(DMC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC), and
dipropyl carbonate (DPC), and cyclic carboxylic acid esters. Among
them, a solvent mixture of a cyclic carbonic acid ester and a
non-cyclic carbonic acid ester is preferably used.
[0063] The lithium salt may be any conventionally known one without
any particular limitation, but LiPF.sub.6, LiBF.sub.4, or the like
is preferably used. It is also possible to use a combination of two
or more kinds of lithium salts.
[0064] The separator interposed between the positive electrode and
the negative electrode is preferably a microporous thin film having
ionic permeability, mechanical strength, and an electron-insulating
property in good balance. It is preferred that the separator have
the function of closing its pores above a certain temperature to
increase internal resistance. The pore size of the separator is
desirably such that the electrode active materials, binder,
conductive agent, etc., that have fallen off the electrodes do not
pass through the pores, and it is, for example, 0.01 to 1 .mu.m.
The thickness of the separator is, for example, 10 to 300 .mu.m.
The porosity of the separator is, for example, 30 to 80%.
[0065] The lithium-ion conductive porous film interposed between
the positive electrode and the negative electrode may be a porous
film comprising a polymer material bonded to the surface of the
positive electrode or negative electrode. Such a porous film can be
formed by applying a mixture of a polymer material and a solvent on
the electrode surface and volatilizing the solvent. The porous film
serves to hold the lithium-ion conductive non-aqueous electrolyte.
Exemplary polymer materials include polyvinylidene fluoride and
vinylidene fluoride-hexafluoropropylene copolymer.
[0066] The present invention is hereinafter described more
specifically by way of Examples.
EXAMPLE 1
(1) Positive Electrode Preparation
[0067] A composite oxide A,
LiCo.sub.0.94Mg.sub.0.05Al.sub.0.01O.sub.2 was used as a positive
electrode active material.
[0068] A positive electrode material mixture paste was prepared by
mixing 100 parts by weight of the positive electrode active
material, 3 parts by weight of acetylene black as a conductive
agent, 5 parts by weight of polyvinylidene fluoride as a binder,
and a suitable amount of N-methyl-2-pyrrolidone.
[0069] Subsequently, the positive electrode material mixture paste
was applied to both sides of a positive electrode current collector
made of a 20 .mu.m-thick aluminum foil, and the applied film was
dried and rolled with rollers, to form a positive electrode
material mixture layer. Thereafter, the positive electrode current
collector with the positive electrode material mixture layers
carried on both sides thereof was cut into a sheet of predetermined
dimensions, to prepare a positive electrode.
[0070] The weight Wp of the positive electrode active material
contained in the positive electrode material mixture layer per unit
area (the positive electrode material mixture layer carried on one
side of the positive electrode current collector per unit area) was
22.8 mg/cm.sup.2.
(2) Negative Electrode Preparation
[0071] Flake graphite with a mean particle size of approximately 20
.mu.m was used as a negative electrode active material.
[0072] A negative electrode material mixture paste was prepared by
mixing 100 parts by weight of the negative electrode active
material, 3 parts by weight of styrene/butadiene rubber as a
binder, 1 part by weight of carboxymethyl cellulose, and a suitable
amount of water.
[0073] Subsequently, the negative electrode material mixture paste
was applied to both sides of a current collector made of a 15
.mu.m-thick copper foil, and the applied film was dried and rolled
with rollers, to prepare a negative electrode material mixture
layer. Thereafter, the negative electrode current collector with
the negative electrode material mixture layers carried on both
sides thereof was cut into a sheet having predetermined dimensions,
to prepare a negative electrode.
[0074] The weight Wn of the negative electrode active material
contained in the negative electrode material mixture layer per unit
area (the negative electrode material mixture layer carried on one
side of the negative electrode current collector per unit area) was
11.4 mg/cm.sup.2.
[0075] The dimensions of the negative electrode were made larger
than those of the positive electrode.
[0076] It should be noted that in the area where the positive
electrode material mixture layer and the negative electrode
material mixture layer are not opposed to each other, the electrode
active materials do not contribute to charge and discharge. Thus,
in this area, there is no need to control the amounts of the
electrode active materials contained in the electrode material
mixture layers per unit area.
(3) Non-Aqueous Electrolyte Preparation
[0077] A non-aqueous electrolyte was prepared by mixing ethylene
carbonate and ethyl methyl carbonate in a weight ratio of 30:70 and
dissolving LiPF.sub.6 at a concentration of 1.0 mol/L in the
resultant solvent mixture.
(4) Electrode Group Fabrication
[0078] The positive electrode sheet and the negative electrode
sheet prepared in the above manner were spirally wound, with a
separator interposed therebetween, to obtain an electrode group.
The separator used was a 25 .mu.m-thick microporous thin film made
of polyethylene resin.
[0079] In this example, in the area where the positive electrode
material mixture layer and the negative electrode material mixture
layer were opposed to each other, the ratio R:Wp/Wn was 2.0 where
Wp is the weight of the positive electrode active material
contained in the positive electrode material mixture layer per unit
opposed area and Wn is the weight of the negative electrode active
material contained in the negative electrode material mixture layer
per unit opposed area.
(5) Battery Fabrication
[0080] Using an electrode group 1 thus obtained, a rectangular
non-aqueous electrolyte secondary battery (thickness 5.2 mm, width
34 mm, height 50 mm), as illustrated in FIG. 1, was produced.
[0081] As illustrated in FIG. 1, one end of an aluminum positive
electrode lead 2 and one end of a nickel negative electrode lead 3
were welded to the positive electrode and the negative electrode,
respectively. An insulating ring made of polyethylene resin was
fitted on the electrode group 1, which was then placed in an
aluminum battery case 4. The other end of the positive electrode
lead 2 was spot-welded to an aluminum sealing plate 5. The other
end of the negative electrode lead 3 was spot-welded to a lower
part of a nickel negative electrode terminal 6 positioned in the
center of the sealing plate 5. The opening end of the battery case
4 was welded by laser to a peripheral part of the sealing plate 5.
A predetermined amount of the non-aqueous electrolyte was injected
therein from an inlet before the inlet was closed with an aluminum
sealing stopper 7. Lastly, the inlet was closed with the sealing
stopper 7, followed by laser welding. This completed a battery
(hereinafter referred to as battery 6).
(6) Production of Various Batteries
[0082] Batteries 1 to 5 and 7 to 9 were produced in the same manner
as the battery 6, except that the ratio R:Wp/Wn was varied in the
area where the positive electrode material mixture layer and the
negative electrode material mixture layer were opposed to each
other, as listed in Table 1.
[0083] Also, batteries 10 to 29 were produced in the same manner as
the battery 6, except that the composite oxides A as shown in Table
1 were used as the positive electrode active materials and the
ratio R:Wp/Wn was varied as shown in Table 1.
[0084] Further, for comparison, batteries A and B were produced in
the same manner as the batteries 6 and 4, except that LiCoO.sub.2
was used alone as the positive electrode active material.
[0085] Table 1 shows the relations between Wp, Wn, and the ratio
R:Wp/Wn of the respective batteries. TABLE-US-00001 TABLE 1 Active
material weight Positive electrode (mg/cm.sup.2) active material R:
Wp/Wn Wp Wn Battery 1 LiCo.sub.0.94Mg.sub.0.05Al.sub.0.01O.sub.2
1.20 18.8 15.7 Battery 2 LiCo.sub.0.94Mg.sub.0.05Al.sub.0.01O.sub.2
1.30 19.3 14.8 Battery 3 LiCo.sub.0.94Mg.sub.0.05Al.sub.0.01O.sub.2
1.40 19.8 14.1 Battery 4 LiCo.sub.0.94Mg.sub.0.05Al.sub.0.01O.sub.2
1.50 20.3 13.5 Battery 5 LiCo.sub.0.94Mg.sub.0.05Al.sub.0.01O.sub.2
1.75 21.5 12.3 Battery 6 LiCo.sub.0.94Mg.sub.0.05Al.sub.0.01O.sub.2
2.00 22.8 11.4 Battery 7 LiCo.sub.0.94Mg.sub.0.05Al.sub.0.01O.sub.2
2.20 23.7 10.8 Battery 8 LiCo.sub.0.94Mg.sub.0.05Al.sub.0.01O.sub.2
2.30 24.3 10.6 Battery 9 LiCo.sub.0.94Mg.sub.0.05Al.sub.0.01O.sub.2
2.40 24.8 10.3 Battery A LiCoO.sub.2 2.00 22.8 11.4 Battery B
LiCoO.sub.2 1.50 20.3 13.5 Battery 10
Li.sub.1.01Co.sub.0.94Mg.sub.0.05Al.sub.0.01O.sub.2 2.00 22.8 11.4
Battery 11 Li.sub.1.01Co.sub.0.94Mg.sub.0.05Al.sub.0.01O.sub.2 1.50
20.3 13.5 Battery 12
Li.sub.1.02Co.sub.0.94Mg.sub.0.05Al.sub.0.01O.sub.2 2.00 22.8 11.4
Battery 13 Li.sub.1.02Co.sub.0.94Mg.sub.0.05Al.sub.0.01O.sub.2 1.50
20.3 13.5 Battery 14
Li.sub.1.03Co.sub.0.94Mg.sub.0.05Al.sub.0.01O.sub.2 2.00 22.8 11.4
Battery 15 Li.sub.1.03Co.sub.0.94Mg.sub.0.05Al.sub.0.01O.sub.2 1.50
20.3 13.5 Battery 16 LiCo.sub.0.985Mg.sub.0.005Al.sub.0.01O.sub.2
2.00 22.8 11.4 Battery 17
LiCo.sub.0.985Mg.sub.0.005Al.sub.0.01O.sub.2 1.50 20.3 13.5 Battery
18 LiCo.sub.0.89Mg.sub.0.1Al.sub.0.01O.sub.2 2.00 22.8 11.4 Battery
19 LiCo.sub.0.89Mg.sub.0.1Al.sub.0.01O.sub.2 1.50 20.3 13.5 Battery
20 LiCo.sub.0.949Mg.sub.0.05Al.sub.0.001O.sub.2 2.00 22.8 11.4
Battery 21 LiCo.sub.0.949Mg.sub.0.05Al.sub.0.001O.sub.2 1.50 20.3
13.5 Battery 22 LiCo.sub.0.9Mg.sub.0.05Al.sub.0.05O.sub.2 2.00 22.8
11.4 Battery 23 LiCo.sub.0.9Mg.sub.0.05Al.sub.0.05O.sub.2 1.50 20.3
13.5 Battery 24 LiCo.sub.0.994Mg.sub.0.005Al.sub.0.001O.sub.2 2.00
22.8 11.4 Battery 25 LiCo.sub.0.994Mg.sub.0.005Al.sub.0.001O.sub.2
1.50 20.3 13.5 Battery 26 LiCo.sub.0.85Mg.sub.0.1Al.sub.0.05O.sub.2
2.00 22.8 11.4 Battery 27 LiCo.sub.0.85Mg.sub.0.1Al.sub.0.05O.sub.2
1.50 20.3 13.5 Battery 28 LiCo.sub.0.88Mg.sub.0.1Al.sub.0.02O.sub.2
2.00 22.8 11.4 Battery 29 LiCo.sub.0.88Mg.sub.0.1Al.sub.0.02O.sub.2
1.50 20.3 13.5
(7) Experimental evaluation <a> Charge/Discharge Cycle
Characteristics
[0086] The charge/discharge cycle of the batteries 1 to 29 and
comparative batteries A and B produced in the above manner was
repeated 500 times at an ambient temperature of 20.degree. C.
[0087] The charging conditions are as follows.
[0088] Constant voltage charge duration: 2 hours
[0089] Maximum current: 600 mA
[0090] End of charge voltage: 4.25 V, 4.4 V, or 4.5 V
[0091] The discharging conditions are as follows.
[0092] Constant current discharge
[0093] Current value: 600 mA
[0094] End of discharge voltage: 3.0 V
[0095] After the 500 cycles of charge/discharge, the discharge
capacity of each battery was measured, and the ratio of this
discharge capacity to the initial discharge capacity was found as
the capacity maintenance rate. Table 2 shows the results.
TABLE-US-00002 TABLE 2 Positive electrode R: End of charge voltage
active material Wp/Wn 4.25 V 4.4 V 4.5 V Battery 1
LiCo.sub.0.94Mg.sub.0.05Al.sub.0.01O.sub.2 1.20 76% 73% 70% Battery
2 LiCo.sub.0.94Mg.sub.0.05Al.sub.0.01O.sub.2 1.30 78% 74% 73%
Battery 3 LiCo.sub.0.94Mg.sub.0.05Al.sub.0.01O.sub.2 1.40 79% 76%
74% Battery 4 LiCo.sub.0.94Mg.sub.0.05Al.sub.0.01O.sub.2 1.50 80%
82% 80% Battery 5 LiCo.sub.0.94Mg.sub.0.05Al.sub.0.01O.sub.2 1.75
81% 81% 80% Battery 6 LiCo.sub.0.94Mg.sub.0.05Al.sub.0.01O.sub.2
2.00 80% 77% 75% Battery 7
LiCo.sub.0.94Mg.sub.0.05Al.sub.0.01O.sub.2 2.20 79% 74% 73% Battery
8 LiCo.sub.0.94Mg.sub.0.05Al.sub.0.01O.sub.2 2.30 70% 64% 59%
Battery 9 LiCo.sub.0.94Mg.sub.0.05Al.sub.0.01O.sub.2 2.40 65% 50%
40% Battery A LiCoO.sub.2 2.00 45% 39% 31% Battery B LiCoO.sub.2
1.50 44% 40% 30% Battery 10
Li.sub.1.01Co.sub.0.94Mg.sub.0.05Al.sub.0.01O.sub.2 2.00 79% 78%
76% Battery 11 Li.sub.1.01Co.sub.0.94Mg.sub.0.05Al.sub.0.01O.sub.2
1.50 81% 81% 80% Battery 12
Li.sub.1.02Co.sub.0.94Mg.sub.0.05Al.sub.0.01O.sub.2 2.00 80% 79%
78% Battery 13 Li.sub.1.02Co.sub.0.94Mg.sub.0.05Al.sub.0.01O.sub.2
1.50 79% 80% 78% Battery 14
Li.sub.1.03Co.sub.0.94Mg.sub.0.05Al.sub.0.01O.sub.2 2.00 80% 77%
76% Battery 15 Li.sub.1.03Co.sub.0.94Mg.sub.0.05Al.sub.0.01O.sub.2
1.50 81% 79% 78% Battery 16
LiCo.sub.0.985Mg.sub.0.005Al.sub.0.01O.sub.2 2.00 75% 73% 72%
Battery 17 LiCo.sub.0.985Mg.sub.0.005Al.sub.0.01O.sub.2 1.50 74%
72% 70% Battery 18 LiCo.sub.0.89Mg.sub.0.1Al.sub.0.01O.sub.2 2.00
80% 81% 80% Battery 19 LiCo.sub.0.89Mg.sub.0.1Al.sub.0.01O.sub.2
1.50 82% 80% 76% Battery 20
LiCo.sub.0.949Mg.sub.0.05Al.sub.0.001O.sub.2 2.00 79% 76% 74%
Battery 21 LiCo.sub.0.949Mg.sub.0.05Al.sub.0.001O.sub.2 1.50 78%
77% 75% Battery 22 LiCo.sub.0.9Mg.sub.0.05Al.sub.0.05O.sub.2 2.00
79% 78% 77% Battery 23 LiCo.sub.0.9Mg.sub.0.05Al.sub.0.05O.sub.2
1.50 80% 79% 78% Battery 24
LiCo.sub.0.994Mg.sub.0.005Al.sub.0.001O.sub.2 2.00 72% 68% 62%
Battery 25 LiCo.sub.0.994Mg.sub.0.005Al.sub.0.001O.sub.2 1.50 76%
73% 70% Battery 26 LiCo.sub.0.85Mg.sub.0.1Al.sub.0.05O.sub.2 2.00
80% 77% 75% Battery 27 LiCo.sub.0.85Mg.sub.0.1Al.sub.0.05O.sub.2
1.50 81% 78% 76% Battery 28
LiCo.sub.0.88Mg.sub.0.1Al.sub.0.02O.sub.2 2.00 79% 76% 75% Battery
29 LiCo.sub.0.88Mg.sub.0.1Al.sub.0.02O.sub.2 1.50 80% 78% 76%
[0096] As can be seen from Table 2, the batteries 1 to 29, which
use the positive electrode active materials containing Mg and Al,
have better capacity maintenance rates after the 500 cycles of
charge/discharge than the comparative batteries A and B, which use
LiCoO.sub.2 as the positive electrode active material. Also, even
in the case of setting the end of charge voltage in
charge/discharge cycles to the high voltage of 4.25 V or higher,
the batteries 1 to 29 maintain high capacity maintenance rates.
[0097] The battery A after the 500 cycles of charge/discharge was
disassembled to collect its positive electrode active material of
LiCoO.sub.2, which was then subjected to an X-ray diffraction
analysis. As a result, it was found that the crystal structure of
LiCoO.sub.2 was largely different from the initial state. This has
confirmed that if charge/discharge is repeated with the end of
charge voltage set to a high voltage, LiCoO.sub.2 deteriorates
significantly.
[0098] On the other hand, the batteries 1 to 9 after the 500 cycles
of charge/discharge were disassembled to collect their positive
electrode active materials of
LiCo.sub.0.94Mg.sub.0.05Al.sub.0.01O.sub.2, which were then
subjected to an X-ray diffraction analysis. As a result, it was
found that the crystal structure of
LiCo.sub.0.94Mg.sub.0.05Al.sub.0.01O.sub.2 maintained the initial
state with a high fraction. This has confirmed that even if
charge/discharge is repeated with the end of charge voltage set to
a high voltage, the crystal structure of
LiCo.sub.0.94Mg.sub.0.05Al.sub.0.01O.sub.2 is stable.
[0099] Also, the batteries 1 to 7 and 10 to 29, whose ratios
R:Wp/Wn were equal to or below 2.2, exhibited better capacity
maintenance rates particularly when the end of charge voltage was
set high than the batteries 8 and 9, whose ratios R were greater
than 2.2.
[0100] The batteries 8 and 9 exhibited no deterioration in the
X-ray diffraction analyses of the crystal structures of their
positive electrodes. However, due to their large ratios R and large
negative electrode loads during charging, their negative electrode
potentials are constantly low. As a result, it has become clear
that due to accumulation of reductive decomposition products of the
non-aqueous electrolyte, the charge/discharge reactions are
inhibited. This indicates that if the ratio R:Wp/Wn exceeds 2.2,
the repetition of the charge/discharge cycles causes an increase in
the resistance to the movement of lithium ions inside the battery,
thereby leading to a gradual degradation of the capacity.
[0101] Also, when the positive electrode active materials used
therein are represented by
Li.sub.xCo.sub.1-y-zMg.sub.yAl.sub.zO.sub.2, even if x, y and z are
varied within the ranges of 1.0.ltoreq.x.ltoreq.1.03,
0.005.ltoreq.y+z.ltoreq.0.15, and 0.001.ltoreq.z.ltoreq.0.05, high
capacity maintenance rates are obtained.
[0102] From the above, it has been confirmed that the batteries
using the predetermined positive electrode active materials exhibit
high charge/discharge cycle characteristics even if they are
repeatedly charged and discharged with the end of charge voltage
set to the high voltage of 4.25 V to 4.5 V. It has also been
confirmed that the batteries whose ratios R:Wp/Wn are controlled in
the predetermined range provide particularly good charge/discharge
cycle characteristics.
<b> Thermorunaway Threshold Temperature
[0103] The batteries whose initial capacities had been checked were
charged up to a predetermined end of charge voltage. The charged
batteries were placed in a temperature controller, and the
threshold temperature leading to thermorunaway was measured by
increasing the battery temperature at 5.degree. C./min.
[0104] The charging conditions are as follows.
[0105] Constant voltage charge duration: 2 hours
[0106] Maximum current: 600 mA
[0107] End of charge voltage: 4.2 V, 4.25 V, 4.4 V or 4.5 V
[0108] Table 3 shows thermorunaway threshold temperatures of the
respective batteries in respective charged states. TABLE-US-00003
TABLE 3 Positive electrode End of charge voltage active material R:
Wp/Wn 4.2 V 4.25 V 4.4 V 4.5 V Battery 1
LiCo.sub.0.94Mg.sub.0.05Al.sub.0.01O.sub.2 1.20 160.degree. C.
154.degree. C. 152.degree. C. 150.degree. C. Battery 2
LiCo.sub.0.94Mg.sub.0.05Al.sub.0.01O.sub.2 1.30 166.degree. C.
162.degree. C. 160.degree. C. 155.degree. C. Battery 3
LiCo.sub.0.94Mg.sub.0.05Al.sub.0.01O.sub.2 1.40 170.degree. C.
166.degree. C. 164.degree. C. 160.degree. C. Battery 4
LiCo.sub.0.94Mg.sub.0.05Al.sub.0.01O.sub.2 1.50 175.degree. C.
173.degree. C. 172.degree. C. 170.degree. C. Battery 5
LiCo.sub.0.94Mg.sub.0.05Al.sub.0.01O.sub.2 1.75 173.degree. C.
171.degree. C. 172.degree. C. 170.degree. C. Battery 6
LiCo.sub.0.94Mg.sub.0.05Al.sub.0.01O.sub.2 2.00 174.degree. C.
173.degree. C. 171.degree. C. 171.degree. C. Battery 7
LiCo.sub.0.94Mg.sub.0.05Al.sub.0.01O.sub.2 2.20 173.degree. C.
172.degree. C. 172.degree. C. 172.degree. C. Battery 8
LiCo.sub.0.94Mg.sub.0.05Al.sub.0.01O.sub.2 2.30 170.degree. C.
162.degree. C. 160.degree. C. 155.degree. C. Battery 9
LiCo.sub.0.94Mg.sub.0.05Al.sub.0.01O.sub.2 2.40 168.degree. C.
158.degree. C. 150.degree. C. 150.degree. C. Battery A LiCoO.sub.2
2.00 162.degree. C. 152.degree. C. 141.degree. C. 135.degree. C.
Battery B LiCoO.sub.2 1.50 160.degree. C. 153.degree. C.
140.degree. C. 136.degree. C. Battery 10
Li.sub.1.01Co.sub.0.94Mg.sub.0.05Al.sub.0.01O.sub.2 2.00
174.degree. C. 173.degree. C. 171.degree. C. 170.degree. C. Battery
11 Li.sub.1.01Co.sub.0.94Mg.sub.0.05Al.sub.0.01O.sub.2 1.50
174.degree. C. 173.degree. C. 171.degree. C. 170.degree. C. Battery
12 Li.sub.1.02Co.sub.0.94Mg.sub.0.05Al.sub.0.01O.sub.2 2.00
174.degree. C. 173.degree. C. 172.degree. C. 171.degree. C. Battery
13 Li.sub.1.02Co.sub.0.94Mg.sub.0.05Al.sub.0.01O.sub.2 1.50
173.degree. C. 173.degree. C. 171.degree. C. 170.degree. C. Battery
14 Li.sub.1.03Co.sub.0.94Mg.sub.0.05Al.sub.0.01O.sub.2 2.00
170.degree. C. 168.degree. C. 166.degree. C. 164.degree. C. Battery
15 Li.sub.1.03Co.sub.0.94Mg.sub.0.05Al.sub.0.01O.sub.2 1.50
171.degree. C. 169.degree. C. 166.degree. C. 165.degree. C. Battery
16 LiCo.sub.0.985Mg.sub.0.005Al.sub.0.01O.sub.2 2.00 170.degree. C.
168.degree. C. 167.degree. C. 163.degree. C. Battery 17
LiCo.sub.0.985Mg.sub.0.005Al.sub.0.01O.sub.2 1.50 172.degree. C.
170.degree. C. 168.degree. C. 165.degree. C. Battery 18
LiCo.sub.0.89Mg.sub.0.1Al.sub.0.01O.sub.2 2.00 175.degree. C.
173.degree. C. 171.degree. C. 170.degree. C. Battery 19
LiCo.sub.0.89Mg.sub.0.1Al.sub.0.01O.sub.2 1.50 178.degree. C.
175.degree. C. 172.degree. C. 171.degree. C. Battery 20
LiCo.sub.0.949Mg.sub.0.05Al.sub.0.001O.sub.2 2.00 173.degree. C.
172.degree. C. 172.degree. C. 170.degree. C. Battery 21
LiCo.sub.0.949Mg.sub.0.05Al.sub.0.001O.sub.2 1.50 174.degree. C.
173.degree. C. 171.degree. C. 170.degree. C. Battery 22
LiCo.sub.0.9Mg.sub.0.05Al.sub.0.05O.sub.2 2.00 172.degree. C.
169.degree. C. 170.degree. C. 168.degree. C. Battery 23
LiCo.sub.0.9Mg.sub.0.05Al.sub.0.05O.sub.2 1.50 173.degree. C.
171.degree. C. 171.degree. C. 170.degree. C. Battery 24
LiCo.sub.0.994Mg.sub.0.005Al.sub.0.001O.sub.2 2.00 168.degree. C.
159.degree. C. 155.degree. C. 150.degree. C. Battery 25
LiCo.sub.0.994Mg.sub.0.005Al.sub.0.001O.sub.2 1.50 169.degree. C.
163.degree. C. 157.degree. C. 152.degree. C. Battery 26
LiCo.sub.0.85Mg.sub.0.1Al.sub.0.05O.sub.2 2.00 176.degree. C.
174.degree. C. 173.degree. C. 171.degree. C. Battery 27
LiCo.sub.0.85Mg.sub.0.1Al.sub.0.05O.sub.2 1.50 178.degree. C.
175.degree. C. 173.degree. C. 172.degree. C. Battery 28
LiCo.sub.0.88Mg.sub.0.1Al.sub.0.02O.sub.2 2.00 178.degree. C.
176.degree. C. 175.degree. C. 174.degree. C. Battery 29
LiCo.sub.0.88Mg.sub.0.1Al.sub.0.02O.sub.2 1.50 178.degree. C.
175.degree. C. 174.degree. C. 173.degree. C.
[0109] As can be seen from Table 3, in the case of comparative
batteries A and B using LiCoO.sub.2 as the positive electrode
active material, when the end of charge voltage was 4.2 V, the
thermorunaway threshold temperature was 162.degree. C., thereby
exhibiting high thermal stability. However, when the end of charge
voltage was increased, the thermorunaway threshold temperature
lowered remarkably, and the safety of the batteries
deteriorated.
[0110] On the other hand, in the case of the batteries 1 to 29,
which used the positive electrode active materials containing Mg
and Al, even when the end of charge voltage was set to the very
high voltage of 4.5 V, they maintained thermorunaway threshold
temperatures of 150.degree. C. or higher. That is, it has been
clearly confirmed that the batteries 1 to 29 have high safety.
[0111] Also, among the batteries 4 to 7 and 10 to 29 whose ratios
R:Wp/Wn were set in the range of 1.5 or more and 2.2 or less, many
of them exhibited thermorunaway threshold temperatures of
170.degree. C. or higher, even if the end of charge voltage was set
to the very high voltage of 4.5 V. That is, it has been clearly
confirmed that the batteries whose ratios R:Wp/Wn were set in the
range of 1.5 or more and 2.2 or less have very high safety.
[0112] In the batteries whose ratios R:Wp/Wn were set to 1.4 or
less, the weight of the negative electrode active material is
greater relative to the weight of the positive electrode active
material, and hence, it is thought that large heat is generated by
the decomposition reaction of the non-aqueous electrolyte by the
negative electrode. This is considered to be the reason why the
safety of these batteries deteriorated slightly. Particularly, the
battery whose ratio R was 1.2 exhibited a large decrease in the
thermorunaway threshold temperature.
[0113] From the above, it has been confirmed that the batteries
using the predetermined positive electrode active materials exhibit
high safety even if they are charged to the high voltage of 4.25 V
to 4.5 V. It has also been confirmed that the batteries whose
ratios R:Wp/Wn were controlled at 1.5 or more and 2.2 or less
exhibit particularly high safety.
[0114] Considering all the factors from Tables 1 to 3, it can be
understood that even if the battery is charged to the high voltage
range of 4.25 to 4.5 V, the use of a predetermined positive
electrode active material and the setting of the ratio R:Wp/Wn in
the range of 1.3 to 2.2 make it possible to realize a battery
having high capacity, excellent charge/discharge cycle
characteristics, and high safety, and further, to realize a charge
and discharge system including such battery. Particularly when the
positive electrode active material is
LiCo.sub.0.94Mg.sub.0.05Al.sub.0.01O.sub.2, it is effective to
control the ratio R in the range of 1.5 to 2.2.
[0115] Next, positive electrode active materials containing Ti, Sr,
Mn, Ni or Ca instead of Mg were prepared, and using the resultant
materials, the same operations as the above were performed. As a
result, essentially the same results were obtained.
[0116] Further, positive electrode active materials containing Ti,
Sr, Mn, Ni or Ca instead of Al were prepared, and using the
resultant materials, the same operations as the above were
performed. As a result, essentially the same results were obtained
as well.
[0117] From the above, it has been confirmed that as long as a
composite oxide represented by Li.sub.xCo.sub.1-yM.sub.yO.sub.2
where the element M is at least one selected from the group
consisting of Mg, Al, Ti, Sr, Mn, Ni and Ca is used as the positive
electrode active material and the ratio R is controlled in the
range of 1.5 to 2.2, it is possible to realize a battery having
high capacity, excellent charge/discharge cycle characteristics,
and high safety, or a charge and discharge system therefor.
EXAMPLE 2
[0118] Batteries 30 to 43 having the relations between Wp, Wn, and
the ratio R:Wp/Wn as listed in Table 4 were produced in the same
manner as in Example 1, except that composite oxides B as listed in
Table 4 were used as the positive electrode active materials. They
were subjected to the same experimental evaluation as that of
Example 1.
[0119] Table 5 shows capacity maintenance rates of the respective
batteries after 500 cycles of charge/discharge. Also, Table 6 shows
thermorunaway threshold temperatures of the respective batteries in
respective charged states. TABLE-US-00004 TABLE 4 Active material
weight Positive electrode (mg/cm.sup.2) active material R: Wp/Wn Wp
Wn Battery 30 LiNi.sub.0.4Mn.sub.0.4Co.sub.0.1Mg.sub.0.1O.sub.2
1.20 18.8 15.7 Battery 31
LiNi.sub.0.4Mn.sub.0.4Co.sub.0.1Mg.sub.0.1O.sub.2 1.30 19.3 14.8
Battery 32 LiNi.sub.0.4Mn.sub.0.4Co.sub.0.1Mg.sub.0.1O.sub.2 1.40
19.8 14.1 Battery 33
LiNi.sub.0.4Mn.sub.0.4Co.sub.0.1Mg.sub.0.1O.sub.2 1.50 20.3 13.5
Battery 34 LiNi.sub.0.4Mn.sub.0.4Co.sub.0.1Mg.sub.0.1O.sub.2 1.75
21.5 12.3 Battery 35
LiNi.sub.0.4Mn.sub.0.4Co.sub.0.1Mg.sub.0.1O.sub.2 2.00 22.8 11.4
Battery 36 LiNi.sub.0.4Mn.sub.0.4Co.sub.0.1Mg.sub.0.1O.sub.2 2.20
23.7 10.8 Battery 37
LiNi.sub.0.4Mn.sub.0.4Co.sub.0.1Mg.sub.0.1O.sub.2 2.30 24.3 10.6
Battery 38 LiNi.sub.0.4Mn.sub.0.4Co.sub.0.1Mg.sub.0.1O.sub.2 2.40
24.8 10.3 Battery 39 LiNi.sub.0.45Mn.sub.0.45Co.sub.0.1O.sub.2 1.50
20.3 13.5 Battery 40 LiNi.sub.0.45Mn.sub.0.45Mg.sub.0.1O.sub.2 1.50
20.3 13.5 Battery 41 LiNi.sub.0.45Mn.sub.0.45Al.sub.0.1O.sub.2 1.50
20.3 13.5 Battery 42 LiNi.sub.0.45Mn.sub.0.45Ti.sub.0.1O.sub.2 1.50
20.3 13.5 Battery 43 LiNi.sub.0.45Mn.sub.0.45Sr.sub.0.1O.sub.2 1.50
20.3 13.5 Battery A LiCoO.sub.2 2.00 22.8 11.4 Battery B
LiCoO.sub.2 1.50 20.3 13.5
[0120] TABLE-US-00005 TABLE 5 Positive electrode R: End of charge
voltage active material Wp/Wn 4.25 V 4.4 V 4.5 V Battery 30
LiNi.sub.0.4Mn.sub.0.4Co.sub.0.1Mg.sub.0.1O.sub.2 1.20 78% 75% 73%
Battery 31 LiNi.sub.0.4Mn.sub.0.4Co.sub.0.1Mg.sub.0.1O.sub.2 1.30
79% 78% 76% Battery 32
LiNi.sub.0.4Mn.sub.0.4Co.sub.0.1Mg.sub.0.1O.sub.2 1.40 80% 82% 80%
Battery 33 LiNi.sub.0.4Mn.sub.0.4Co.sub.0.1Mg.sub.0.1O.sub.2 1.50
82% 81% 81% Battery 34
LiNi.sub.0.4Mn.sub.0.4Co.sub.0.1Mg.sub.0.1O.sub.2 1.75 81% 81% 80%
Battery 35 LiNi.sub.0.4Mn.sub.0.4Co.sub.0.1Mg.sub.0.1O.sub.2 2.00
79% 75% 74% Battery 36
LiNi.sub.0.4Mn.sub.0.4Co.sub.0.1Mg.sub.0.1O.sub.2 2.20 76% 75% 72%
Battery 37 LiNi.sub.0.4Mn.sub.0.4Co.sub.0.1Mg.sub.0.1O.sub.2 2.30
70% 68% 62% Battery 38
LiNi.sub.0.4Mn.sub.0.4Co.sub.0.1Mg.sub.0.1O.sub.2 2.40 66% 55% 43%
Battery 39 LiNi.sub.0.45Mn.sub.0.45Co.sub.0.1O.sub.2 1.50 80% 75%
74% Battery 40 LiNi.sub.0.45Mn.sub.0.45Mg.sub.0.1O.sub.2 1.50 79%
75% 74% Battery 41 LiNi.sub.0.45Mn.sub.0.45Al.sub.0.1O.sub.2 1.50
78% 76% 74% Battery 42 LiNi.sub.0.45Mn.sub.0.45Ti.sub.0.1O.sub.2
1.50 79% 74% 73% Battery 43
LiNi.sub.0.45Mn.sub.0.45Sr.sub.0.1O.sub.2 1.50 77% 73% 72% Battery
A LiCoO.sub.2 2.00 45% 39% 31% Battery B LiCoO.sub.2 1.50 44% 40%
30%
[0121] TABLE-US-00006 TABLE 6 Positive electrode End of charge
voltage active material R: Wp/Wn 4.2 V 4.25 V 4.4 V 4.5 V Battery
30 LiNi.sub.0.4Mn.sub.0.4Co.sub.0.1Mg.sub.0.1O.sub.2 1.20
174.degree. C. 173.degree. C. 171.degree. C. 171.degree. C. Battery
31 LiNi.sub.0.4Mn.sub.0.4Co.sub.0.1Mg.sub.0.1O.sub.2 1.30
173.degree. C. 171.degree. C. 172.degree. C. 170.degree. C. Battery
32 LiNi.sub.0.4Mn.sub.0.4Co.sub.0.1Mg.sub.0.1O.sub.2 1.40
175.degree. C. 173.degree. C. 171.degree. C. 170.degree. C. Battery
33 LiNi.sub.0.4Mn.sub.0.4Co.sub.0.1Mg.sub.0.1O.sub.2 1.50
174.degree. C. 171.degree. C. 170.degree. C. 170.degree. C. Battery
34 LiNi.sub.0.4Mn.sub.0.4Co.sub.0.1Mg.sub.0.1O.sub.2 1.75
173.degree. C. 172.degree. C. 171.degree. C. 170.degree. C. Battery
35 LiNi.sub.0.4Mn.sub.0.4Co.sub.0.1Mg.sub.0.1O.sub.2 2.00
173.degree. C. 172.degree. C. 171.degree. C. 172.degree. C. Battery
36 LiNi.sub.0.4Mn.sub.0.4Co.sub.0.1Mg.sub.0.1O.sub.2 2.20
170.degree. C. 162.degree. C. 160.degree. C. 155.degree. C. Battery
37 LiNi.sub.0.4Mn.sub.0.4Co.sub.0.1Mg.sub.0.1O.sub.2 2.30
168.degree. C. 158.degree. C. 150.degree. C. 150.degree. C. Battery
38 LiNi.sub.0.4Mn.sub.0.4Co.sub.0.1Mg.sub.0.1O.sub.2 2.40
165.degree. C. 160.degree. C. 150.degree. C. 154.degree. C. Battery
39 LiNi.sub.0.45Mn.sub.0.45Co.sub.0.1O.sub.2 1.50 173.degree. C.
172.degree. C. 171.degree. C. 170.degree. C. Battery 40
LiNi.sub.0.45Mn.sub.0.45Mg.sub.0.1O.sub.2 1.50 172.degree. C.
172.degree. C. 171.degree. C. 171.degree. C. Battery 41
LiNi.sub.0.45Mn.sub.0.45Al.sub.0.1O.sub.2 1.50 171.degree. C.
172.degree. C. 171.degree. C. 170.degree. C. Battery 42
LiNi.sub.0.45Mn.sub.0.45Ti.sub.0.1O.sub.2 1.50 172.degree. C.
172.degree. C. 170.degree. C. 169.degree. C. Battery 43
LiNi.sub.0.45Mn.sub.0.45Sr.sub.0.1O.sub.2 1.50 170.degree. C.
171.degree. C. 168.degree. C. 165.degree. C. Battery A LiCoO.sub.2
2.00 162.degree. C. 152.degree. C. 141.degree. C. 135.degree. C.
Battery B LiCoO.sub.2 1.50 160.degree. C. 153.degree. C.
140.degree. C. 136.degree. C.
[0122] As can be seen from Tables 4 to 6, the batteries 30 to 43
exhibited excellent charge/discharge cycle characteristics and
safety. Also, the batteries 31 to 35 and 39 to 43, whose ratios
R:Wp/Wn were in the range of 1.3 to 2.0, exhibited high safety even
when they were charged to the high voltage of 4.25 V to 4.5.
[0123] Next, positive electrode active materials containing Al, Ti,
Sr or Ca instead of Mg were prepared, and using the resultant
materials, the same operations as the above were performed. As a
result, essentially the same results were obtained.
[0124] Further, positive electrode active materials containing Al,
Ti, Sr or Ca instead of Co were prepared, and using the resultant
materials, the same operations as the above were performed. As a
result, essentially the same results were obtained as well.
[0125] From the above, it has been confirmed that as long as a
composite oxide represented by
Li.sub.xNi.sub.yMn.sub.zM.sub.1-y-zO.sub.2 where the element M is
at least one selected from the group consisting of Mg, Al, Ti, Sr,
Mn, Ni and Ca is used as the positive electrode active material and
the ratio R is controlled in the range of 1.3 to 2.0, it is
possible to realize a battery having high capacity, excellent
charge/discharge cycle characteristics, and high safety, or a
charge and discharge system therefor.
INDUSTRIAL APPLICABILITY
[0126] As described above, the present invention can provide a
high-capacity non-aqueous electrolyte secondary battery that
operates normally even if the end of charge voltage is set to 4.25
to 4.5 V in normal operation. Also the non-aqueous electrolyte
secondary battery of the present invention can exhibit excellent
charge/discharge cycle characteristics and maintain high safety
even if it is used in the high voltage range of 4.25 to 4.5 V.
* * * * *